This increase in current also causes the dc operating point, sometimes called the quiescent or static point, to move away from its desired position (level). If the temperature of the transistor rises for any reason (due to a rise in ambient temperature or due to current flow through it), collector current will increase. Unfortunately, this simple arrangement is quite thermally unstable. As you recall, it consisted basically of a resistor (Rb) connected between the collector supply voltage and the base. The first biasing method, called BASE CURRENT BIAS or sometimes FIXED BIAS, was used in the previous two figures. Although there are numerous biasing methods, only three basic types will be considered. As mentioned earlier, various biasing methods can be used to accomplish both of these functions. Thus a need arises for a method to properly bias the transistor amplifier and at the same time stabilize its dc operating point (the no signal values of collector voltage and collector current). This is accomplished by selecting the proper circuit-biasing conditions and ensuring these conditions are maintained despite variations in ambient (surrounding) temperature, which cause changes in amplification and even distortion (an unwanted change in a signal). One of the basic problems with transistor amplifiers is establishing and maintaining the proper values of quiescent current and voltage in the circuit.
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